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Petrology and mineral chemistry of sulphide ores and associated metalliferous rocks of the Gamsberg Zn-Pb deposit, South Africa : implications for ore genesis and mineral exploration

Stalder, Marcel (2004-12)

Thesis (PhD)--University of Stellenbosch, 2004.

Thesis

ENGLISH ABSTRACT: The Gamsberg Zn-Pb deposit is a metamorphosed and multiply deformed sediment-hosted base metal
deposit in the central Namaqua Province of South Africa. The deposit is hosted by the Bushmanland
Group, a late Palaeoproterozoic (2000-1600 Ma) supracrustal succession of quartzite, metapelitic schist
and interbedded metavolcanic rocks. Mineralisation occurs within the central part of the Gams Formation,
a heterogeneous sequence of metamorphosed metalliferous sediments and fine-grained organic-rich
shales. The ore horizon is subdivided into a lower unit of metapelite-hosted ore, an intermediate layer
of phosphorite-hosted ore, and an upper unit of banded garnet-apatite ore. The ore body is enveloped by
unmineralised silicate-, carbonate- and oxide-facies metalliferous rocks, which originally represented
mixtures of Fe-Mn-rich hydrothermal precipitates, authigenic carbonate, and variable concentrations of
detrital material. Based on mineralogical and geochemical characteristics, the metalliferous host rocks are
subdivided into iron formations, coticules, Fe-Mn silicates, impure marbles and barite/Ba-rich quartzite.
Minerals of the Gams Formation mostly represent solid solution between the Fe and Mn end-members
of garnet, pyroxene, pyroxenoid, amphibole, olivine, spinel and ilmenite. Calcium-rich rock types are a
typical feature and characterized by the occurrence of manganoan calcite, clinopyroxene, andradite-rich
garnet and titanite. A successive increase in the (Mn+Ca):Fe value of rocks and minerals is evident with
increasing distance from the ore horizon. Amphibole is restricted to Fe-rich ore-bearing assemblages,
whereas orthopyroxene, clinopyroxene, Fe-rich pyroxenoid and olivine are present in intermediate
assemblages, and Mn-rich rhodonite and pyroxmangite in the most manganiferous assemblages. These
variations are mimicked by an increase in the Mn:Fe value of coexisting garnet and ilmenite group
minerals with increasing distance from ore. LA-ICP-MS analyses have been used to constrain the REE
patterns of garnet and apatite. In the ore-body, these minerals display a positive Eu anomaly, which is
interpreted to reflect a distinct hydrothermal signature. In contrast, garnet and apatite in unmineralised
metalliferous rocks display nil or a negative Eu anomaly.
Primary features of the Gams Formation, such as REE patterns, the banded nature of garnet-apatite ore,
the presence of diagenetic apatite nodules, and the distribution of the redox-sensitive elements Ba and
Mn have been used to constrain palaeo-environmental conditions. The results indicate that metapelitehosted
ore has been deposited in a stratified ocean that was characterised by anoxic bottom waters and
precipitation of Fe and Zn sulphides into organic matter-rich shales. These rocks were superceded by
phosphorite-hosted ore, garnet-apatite ore and metalliferous host rocks that developed in a suboxic to
oxic environment. The large size of the deposit, the internal lamination of the ores and the predominance
of sphalerite and barite are consistent with a vent-distal setting and precipitation of the ore-forming
constituents from dense and reduced hydrothermal fluids, which originated due to reactivation of dormant
growth faults. Collectively, the geological evidence indicates that Gamsberg is bridging the gap betweenthe SEDEX and BHT classifications. The relationships demonstrate that differences between these two
classes of sediment-hosted Zn-Pb deposits are predominantly related to environmental conditions within
localised third order basins and not to fundamental differences in ore-forming processes.

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